One known NOx sensor is configured as a flat plate multilayer ceramic package design that includes two or more chambers. In the first chamber there are electrodes attached to an oxygen ion conducting electrolyte membrane, thereby forming an oxygen pump to remove the oxygen. In addition, NO2 is decomposed to NO and one-half O2. The free oxygen is removed in the first chamber so that theoretically the only gas that enters the second chamber is NO. Another oxygen pump is in the second chamber and is a NO decomposing element that removes the oxygen from the NO. The electrical current produced from the decomposition of NO and the transport of oxygen is correlated to the NO concentration.
There are a number of concerns that affect the commercial application of this known NOx sensor. For example, when the NOx concentration to be detected is low, there is significant interference from the residual oxygen. In addition, the signal current is very small, thus making it susceptible to electronic noise commonly found in an automobile. Also, the exhaust gas typically has pulsations in the flow rate caused by cylinder firings that influence the ability of the oxygen pump to effectively remove all of the free oxygen and may result in measurement error. This device may also contain a small diffusion hole that limits the passage of gas into the measurement chambers and is prone to clogging.
Another known NOx sensor utilizes a similar flat plate multilayer ceramic package design. There are a few significant differences in the operation principle for this sensor; namely, the sensor is a mixed potential type rather than amperometric, and the use of the first chamber is for converting NO to NO2 and vice versa. It is a well established phenomenon of mixed potential NOx sensors that the voltage signal generated from the gas species NO and NO2 are of opposite sign, thereby making it difficult to distinguish a meaningful voltage signal in the presence of both gases. Some sensors have attempted to overcome this problem by utilizing the flat plate multilayer package type design with two separate chambers built into the design. Attempts have also been made to convert all of the NOx gas species into a single species with the use of an electrochemical oxygen pump that pumps oxygen into the first chamber—thereby converting all of the gas to NO2—or conversely by removing oxygen from the chamber and reducing all of the NO2 to NO. This conditioned gas then passes into the second chamber where the NOx concentration is measured by the voltage signal generated from a mixed potential type sensor.
There are a number of limitations to this approach that have hampered the commercialization of this configuration. One significant concern is the reproducibility of the conversion system to completely convert all the NOx gases into a single species under varying gas concentration conditions. In addition, the oxygen pump conversion cell tends to degrade with time, further contributing to the issue of reproducibility. Because the effects of these concerns are magnified in the low concentration range, this measurement approach is not well suited for detecting low concentrations of NOx gases.
Additional drawbacks common to both of the sensor mechanisms disclosed above stem from the fundamental design of the flat plate ceramic multilayer system. Response times tend to be slow because of the complexity of the device where gas first enters a diffusion port, is conditioned in a first chamber, and then diffuses into a second chamber. Achieving rapid gas exchange that can keep up with the dynamic environment of the engine exhaust is difficult to achieve in these configurations. Also, the corrosiveness of the gas—along with fine particulates—may result in the clogging of the diffusion controlling port, or at the very least, changes in the gas flow dynamics with time. Finally, the pulsations in the gas flow rates due to cylinder firings and the accompanying electrical noise typical of automobiles make it difficult to control and monitor the low voltage and current circuits associated with these devices.
Another known NOx sensor utilizes a zeolite catalyst to condition the gas prior to being measured by the sensor. Although this catalyst has been demonstrated to be effective in controlled gas environments, no data has been reported wherein the catalyst has suitably performed in H2O containing gases. Exhaust gases from combustion processes such as diesel exhaust always contain some H2O vapor as this is one of the major chemical byproducts of combustion of hydrocarbon fuels along with CO2. As such, the utilization of the NOx sensor incorporating a zeolite catalyst in such applications is limited because of the catalyst's well known instability in the presence of H2O.
The present invention is provided to address these and other considerations.